Three types of models for K_ATP channel gating. Schematic representation of the different types of models. (A) Four independent gating units (4GU₁), each formed by one Kir6.2 subunit. (B) One gating unit formed by all four Kir6.2 subunits (GU₄). (C) Two independent gating units, each formed by two Kir6.2 subunits (2GU₂)_. In each case, the transitions between open and closed states are shown for a single gating unit. Each Kir6.2 subunit can be either in an “open” (circle) or “closed” (square) conformation. It is assumed that ATP binds independently to all subunits and A indicates the presence of bound ATP. K_O and K_C are the binding constants to the open and closed states, respectively. E_O is the equilibrium gating constant in the absence of ATP. The term t indicates the change in the equilibrium gating constant due to bound ATP. It is assumed that ATP binding has an additive effect on the energy of the gating unit in both open and closed conformations.